It is known to provide a vehicle comprising a four stroke internal combustion engine with a ‘stop/start’ functionality. By this is meant that for reasons of fuel efficiency and reducing undesirable emissions, whilst a vehicle may for example be stationary and the running of an ICE/engine of the vehicle is unnecessary, the engine may be automatically stopped provided certain conditions are met. Often, conditions for such stopping include the vehicle being stationary and a brake system being applied by a driver. This equates to the ‘stop’ part of the functionality. Allied to this is a ‘start’ functionality whereby the engine of the vehicle is automatically restarted upon a change to various conditions, such as release of a brake system by a driver of the vehicle, or a requirement to generate torque in order to produce power for another system of the vehicle, such as an air conditioning system. Such systems, and the various means by which stop/start events are triggered by various conditions, are known, and are not discussed further in this application.
Often a ‘stop/start’ event will occur at a given point during a journey such as a momentary cessation of vehicle movement at, for example, a set of traffic lights, or in heavy traffic conditions where a vehicle may progress for a short distance and then stop for a few seconds, then progress a further short distance. In such circumstances, and given the relatively short period of time between an ‘engine stop’ command and a subsequent ‘engine restart’ command associated with such conditional uses of the stop/start system, it is a preferential attribute of the engine in conjunction with the stop/start system that the engine is able to shut down and restart in as expeditious a manner as possible. This then maximises the amount of time during which the engine is stopped, which in turn maximises the zero fuel use period of the engine, thus maximising the benefit of the system with regards to emissions reduction.
Achieving a smooth and fast stop/start is desirable. However in some circumstances the engine may shake or shudder when stopping, due to large changes in acceleration and deceleration of the crankshaft as rotation ceases; the crankshaft may for example make a small reversal of rotation.
The engine may require to be re-started during a stopping event, in which case such reverse rotation may conflict with rotation of a starter motor and result in pinion clash. For example if such an event occurs at less than 400 rpm, a starter motor delay of 0.4 s may be required to obviate pinion clash; this delay is noticeable, and should be eliminated if possible.
The time to cessation of rotation may also vary according to the stop position of the crankshaft and the in-cylinder pressure(s). Re-start time may be influenced by the air mass in the engine cylinders, and by leakage of in-cylinder pressure from a cylinder on a compression stroke. Restart time may also be influenced by the timing of a fuel injection having regard to crankshaft position, in particular of a cylinder which is close to TDC.
Prior art methods and apparatus have proposed management of throttle position during engine stopping to control the air charge to the cylinders, but these may not be very effective for inlet manifolds having a comparatively large volume. In any event prior solutions tend to be a compromise between engine refinement, restart performance and control of engine emissions.
In one prior art system, an engine is shut down by cutting fuel to the engine and closing the throttle, which controls the amount of air entering the inlet manifold. Another way is to cut the fuel whilst leaving the throttle open. Each of these methods results in differing shutdown characteristics.
A ‘closed throttle’ shutdown tends to result in an engine shutdown with comparatively good NVH (noise, vibration and harshness) characteristics compared to an ‘open throttle’ shutdown. One reason for this is that reduced cylinder compression may reduce shutdown shake, which in turn may be influenced by the timing of closing of the throttle, and the respective volume of cylinder and inlet manifold.
Closing the throttle on shutdown allows the engine to continue to rotate before finally stopping, and the valves on the cylinders continue to allow air into the cylinders on their induction stroke. This potentially results in a pressure drop in the inlet manifold. Upon a restart demand on the engine, pressure in the inlet manifold is therefore low, and comparatively low charges of fresh air enter into the engine cylinders until this pressure is regained. This may result in a restart with poor characteristics, such as an undesirably long cranking time before ignition, and a delay in net torque output availability. The time taken for the engine to cease rotation with a closed throttle shutdown may be longer than that when an equivalent ‘open throttle’ shutdown is made.
With an ‘open throttle’ shutdown, inlet manifold pressure is maintained or rises as the engine rotates. Relatively high mass charges of fresh air enter the engine cylinders, and an open throttle shutdown may have a different time than a closed throttle shutdown, depending on the pumping loss and compression loss within a particular engine.
An advantage of the open throttle shutdown is that once the engine has come to a halt, at least one cylinder will tend to be charged with a relatively normal full pressure air charge. Also the inlet manifold is still at full pressure. This means that when there is a restart demand, the restart characteristics may be comparatively good—restart time, and time until net torque output availability, may be shorter. The same advantage applies where a restart demand is made prior to the engine ceasing rotation (a so-called change of mind (COM) event), provided that pinion clash is avoided.
However, the open throttle shutdown tends to result in a comparatively poor NVH performance on shutdown. The effect of the induction, compression and expansion of comparatively high air charges in the engine cylinders as the engine is slowing down is that rotation of the crankshaft becomes irregular or ‘lumpy’, particularly as the engine approaches a final stop. Speed oscillation is large because of the comparatively high forces due to compression of comparatively high air changes in the cylinders. Ultimately the engine may reach a point just prior to stop where the piston in one cylinder has just passed TDC (top dead centre) and has a full compressed air charge, which is acting to continue rotation of the engine, whilst another cylinder has a piston just after BDC (bottom dead centre) and has a full air charge that continued rotation would act to compress and thus resist rotation. At this point the two cylinders act in opposition to each other and this may result in a ‘rocking’ motion with the crankshaft rotationally ‘bouncing’ forwards and backwards until rotation ceases. This is felt by a driver as a further engine shake or vibration. In combination with the ‘lumpy’ irregular crankshaft rotation as the engine slows down, this is an undesirable NVH trait, particularly for higher-end or luxury motor vehicles in which smoothness and quietness are a desirable attribute.
Accordingly it can be seen that in the prior art there is a trade-off to be made between a ‘closed throttle’ and ‘open throttle’ shutdown for stop/start purposes, with each option offering advantages over the other but suffering from relative disadvantages.
What is required is a shutdown procedure or method which offers more of the advantages of the prior art methods, so as to give a low-NVH shutdown whilst preferably also allowing for a fast and effective restart, when a vehicle undertakes a stop/start operation.
More recently, engines have come to be equipped with ‘variable valve’ systems in which active tappets provide for substantially immediate change of operation of an associated valve, independent or at least semi-independent of a camshaft or other valve control device, on a stroke—by—stroke basis. Such an active tappet may include a hydraulic chamber whose volume is controlled by an electrically actuated valve, such as a bleed valve, responsive to a command from an engine ECU. Such a tappet may comprise a solenoid.
Typically such variable valve systems provide for the use of active tappets to vary the size of inlet aperture and/or timing of admission of air into a combustion chamber and the size of exhaust aperture and/or timing of the exhaust of air or combustion gas from the combustion chamber at each stroke of the respective valve. In the prior art this is done typically so as to provide the required charge of air or/and gas in the combustion chamber whilst an engine is running, to provide control of torque and/or to improve the fuel efficiency and/or emissions characteristics of the engine.
In typical standard prior art applications, air charge may be controlled by an active tappet, according to one or more of the following techniques:                varying valve lift so as to increase or decrease the maximum opening of the poppet valve during an activation cycle. If the opening and closing timing is unchanged, an increased lift will increase the mass of aspirated air, and a reduced lift will reduce the mass of aspirated air (if the valve is within the throttling range).        varying the duration of valve opening, either by re-timing valve opening, re-timing valve closing, or both. If the valve lift is unchanged, a longer open duration can be used to increase the mass of aspirated air, and a shorter duration can be used to reduce the mass of aspirated air.        varying the timing overlap of inlet and exhaust valves, by for example re-timing the opening of the inlet valve to increase or reduce overlap with operation of the exhaust valve.        
Reduced overlap will tend to increase the mass of air available for combustion, whereas increased overlap will tend to reduce the mass of air available for combustion.
The mass of air available for combustion may be reduced by directly reducing the mass of a fresh air charge, or by controlling valve overlap to retain a greater proportion of combustion gases within a combustion chamber; such gases are inert and cannot contribute towards combustion.
If an active tappet is also provided for an exhaust valve of the combustion chamber, valve overlap may be varied by means of the inlet valve tappet, the exhaust valve tappet, or both.
The apparatus and method of dealing with stopping and starting of an engine during a stop/start event should preferably be applicable to both diesel and gasoline variants, and accordingly should be susceptible of variation to suit the different stopping and starting characteristics thereof.